This invention relates to a control valve arrangement for a servomotor which provides a power assist to an operator input to effect a brake application in a vehicle. Such servomotors usually derive power through the creation of a force developed across a movable wall by a pressure differential between a partial vacuum on one side of the wall and air at atmospheric pressure on the other side. Air is presented to the wall through the movement of a valve, such as the poppet valve shown in U.S. Pat. No. 3,289,547 or the slide valve shown in U.S. Pat. No. 3,106,873, in a bore of a hub member in response to an operator input.
Poppet valves require seats for both vacuum and air passages to be established on the housing of the hub to assure that partial vacuum and air presented to the wall are independent of each other when an input force is applied to the plunger to effect a brake application.
While the slide valve eliminates a seat on the hub, a seat is still required on the plunger which engages a resilient disc to regulate the communication of air to the wall.
Unfortunately, in both the poppet valve and the slide valve, the seating surfaces on the housing of the hub requires machining which adds to the production costs of the servomotor. If the seating surfaces are not substantially smooth, as leak path can occur whereby air can be presented to the wall to create a pressure differential in the absence of an input force or to both sides of the wall and thereby eliminate or greatly reduce the development of the pressure differential in response to an input force.